KR20140040720A - Optical sensor - Google Patents

Abstract

An apparatus comprising a measuring interferometer 7 and a reference interferometer 3 is disclosed. Each interferometer is configured to receive light from the same light source 1 and to emit light to the respective detectors 6 and has a respective operating point. The measurement interferometer 7 is configured to respond to changes in physical parameters by changing the intensity of light emitted, while the reference interferometer 3 is configured not to respond to changes in physical parameters. The apparatus further comprises a signal processor for generating a differential output signal in accordance with the respective output signals generated by the detectors 6.

Description

Optical sensor {OPTICAL SENSOR}

The present invention relates to an apparatus comprising a measurement interferometer that can be used to form an optical sensor. The invention also relates to a layered structure which can constitute such a device, and an optical microphone comprising such a device.

Various types of optical sensors use interferometers to detect changes in physical parameters. In such sensors, light from the laser is coupled inside the interferometer, where the interferometer is affected by changes in the physical parameters to produce a corresponding change in the interference pattern. This change in interference pattern results in a change in intensity that can be detected by the photo detector.

Various different physical parameters can be used to cause a change in the interference pattern, and thus can be sensed by this type of sensor. Examples include pressure (including air pressure), strain, and displacement.

The signal-to-noise ratio (SNR) of such sensors is often limited by noise caused by variations in the frequency or intensity of the light output from the laser. There are various ways to stabilize the laser frequency. For example, one method uses a Fabry-Perot interferometer or etalon to generate an error signal. Etalons convert frequency fluctuations into intensity fluctuations that can be detected by a light detector. The final photocurrent is used as a feedback signal, for example, to move the cavity mirror or act on the laser supply current to correct for frequency variations.

However, devices of this kind are bulky and expensive. This is not compatible with many applications where the use of an optical sensor may be desirable. For example, in a mobile communication device, a microphone is required which is very stable, shock resistant and insensitive to wind noise. All of these features can be provided by an optical microphone using a laser light source and an interferometer because they do not have a moving part such as a membrane. However, such microphones are also required to be compact and have high SNR. The aforementioned stabilization technique can compensate for laser frequency noise up to a predetermined range, but cannot compensate for relative intensity noise.

According to the invention, there is provided an apparatus comprising a measurement interferometer and a reference interferometer, each of which is configured to receive light from the same light source and to emit light to the respective detectors, each having a working point, the measurement interferometer The reference interferometer is configured not to respond to changes in the physical parameter by varying the intensity of light being made, while the reference interferometer is configured not to respond to changes in the physical parameter, and the device generates a differential output signal in accordance with the respective output signals generated by the detectors. It further comprises a signal processor for generating.

Light from the same light source is coupled inside both the reference interferometer, which is not sensitive or responds to changes in the physical parameters to be sensed, and the measurement interferometer responding to these changes. Therefore, both interferometers will be affected by variations in light from the light source, but only measurement interferometers will be affected by changes in physical parameters; The reference interferometer is isolated from this change. Thus, the interference pattern of the reference interferometer represents only noise from the light source as well as the signal from each detector. This signal can be used to compensate for noise from the light source within the signal from each detector of the measurement interferometer, whereby the signal only represents a change in the physical parameter. Therefore, because the SNR increases and the only additional component required is the reference interferometer, the device can be made very compact.

The operating point of each of the reference interferometer and the measuring interferometer is selected to achieve a linear change in the transmittance of the interferometer with frequency. FIG. 5 shows a graph of transmittance for parameter q (normalized to the maximum value of 1), where q = 4πnd / λ (n is the refractive index in the interferometric cavity; d is the spacing between interferometer mirrors; Wavelength of light). The parameter q is of course proportional to the frequency of the light. The relationship between the transmittance and q is represented by a so-called "airy function". 5 also shows graphs of the first and second derivatives of the transmittance versus q graph. The best linearity of transmittance versus q is found at the point where the second derivative is zero. Thus, the point at which the second derivative of the relationship between transmittance and q is preferably selected as the operating point for each interferometer.

Since small changes in the size of the interferometers will affect the optimal position of the operating points, the operating points are likely to be different for each interferometer.

An alternative operating point that can be used is to tune the interferometers so that the value of transmission at the operating point is 75% of the maximum value. This point is approximately equal to the point where the second derivative is zero.

The operating point can be adjusted by changing any of the parameters on which q depends. Thus, the reference interferometer or measurement interferometer can be tuned to the operating point by adjusting the spacing between mirrors of the cavity or the index of refraction in the interferometer cavity. In addition, the wavelength of the light emitted by the light source can be adjusted to suit the inherent or adjusted operating point of the reference interferometer or measurement interferometer.

The reference interferometer may be configured to not respond to changes in physical parameters by isolating the reference interferometer from physical parameters, or by introducing a cavity in the interferometer, or by filling a cavity in the interferometer with a solid light-transmitting material such as glass. .

Typically, the measurement interferometer and / or reference interferometer is a Fabry-Perot interferometer.

The measurement interferometer and / or the reference interferometer may comprise a pair of spaced mirrors. The mirrors may all be planar mirrors or curved mirrors, or may include one planar mirror and one curved mirror.

For one interferometer, the operating point can be adjusted by adjusting the laser wavelength (eg, by adjusting the supply current to the laser). For another interferometer, a tuning mechanism may be provided.

Therefore, the apparatus may further comprise a thermal tuning element for tuning the operating point of the measuring interferometer or reference interferometer. It utilizes thermo-optical effects to thermally adjust the refractive index of the optical medium of the measurement interferometer or reference interferometer.

The apparatus may further comprise a tuning electrode for tuning the operating point of the measurement interferometer or reference interferometer. This can be done by an externally applied electric field to adjust the index of refraction of the optical medium of the measuring interferometer or reference interferometer (e.g., linear effects such as the Pockels effect or nonlinear effects such as the Kerr effect). Effect).

The apparatus may further comprise a liquid crystal tuning element disposed between the measurement interferometer or reference interferometer and each detector.

In one embodiment, the apparatus further comprises a light source controller configured to cause the light source to emit light alternately at the first and second wavelengths, wherein the operating points of the measurement interferometer and the reference interferometer are respectively at the first and second wavelengths. Is achieved. In this way, the reference interferometer and the measurement interferometer are tuned to the operating points in each alternating operating cycle by adjusting the wavelength of the light source. This can be achieved, for example, by adjusting the supply current for the light source. Therefore, it is possible to omit additional tuning elements when using this "pulse-type operating mode".

Typically, the device further comprises an optical isolator disposed between the light source and the reference interferometer and comprising a linear polarizer and a quarter wave plate.

Preferably, the light source is a laser.

The laser may advantageously be a double-emitting laser, with the first and second emission beams coupled to the measurement interferometer and the reference interferometer, respectively.

The use of double-emitting lasers makes it possible to produce particularly compact structures. In this embodiment, the double-emitting laser is a double-sided emission laser diode disposed on a substrate between the first and second layered structures, each of the first and second layered structures forming one of a measurement interferometer and a reference interferometer, respectively Two respective mirror layers spaced by spacer layers of and a detector layers remote from the substrate with respect to the mirror layers.

Each spacer layer for the reference interferometer will typically include a cavity between two respective mirror layers, which are acoustically coupled to the surrounding environment by holes in the spacer layer.

Each of the first and second layered structures may further include light isolation layers disposed between the substrate and the innermost mirror layer of the respective mirror layers. Optical isolators typically include a linear polarizer and a quarter wave plate.

Each of the first and second layered structures may further comprise a lens disposed between the substrate and the innermost mirror layer of the respective mirror layers. The lens may be disposed between the light isolation layers and the innermost mirror layer of each mirror layer.

Each detector layer may comprise a respective photo detector.

Typically, the signal processor further includes an adaptive equalizer for equalizing the average amplitude of each output generated by the detectors over the equalization period. This ensures that noise is properly canceled out even if the optical power received by the two detectors changes for some reason.

According to a second aspect of the invention there is provided an optical microphone comprising an apparatus according to the first aspect of the invention, wherein the measurement interferometer is acoustically coupled with the surrounding environment and the reference interferometer is acoustically isolated from the surrounding environment. The physical parameter is air pressure.

The “acoustical isolation” of a reference interferometer is a solid state interferometer that separates the reference interferometer from changes in air pressure (eg, disallows fluid communication between the cavity of the reference interferometer and the surrounding environment), or is naturally separated from changes in air pressure. It can be achieved by using a.

Such microphones are very compact and are particularly suitable for mobile communication applications with high SNR.

Typically, the measurement interferometer is acoustically coupled with the surrounding environment by holes in its cavity.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
1 shows a schematic view of a device according to the invention.
2 shows a schematic diagram of an optical microphone.
3A and 3B show cross-sectional views of the measurement interferometer in the optical microphone of FIG. 2.
4 shows a cross-sectional view of a layered structure that can constitute such an optical microphone.
5 shows the transmission function of the interferometer.

FIG. 1 shows a laser light source 1, for example a laser diode, which emits light which travels through the optical isolator 2 to the reference Fabry-Perot interferometer or the etalon 3. The reference Fabry-Perot etalon 3 acts as a frequency discriminator. It may be an air-spaced etalon or evacuated etalon or a solid etalon isolated from the environment.

Tuning electrodes 4 are arranged adjacent to the reference Fabry-Perot etalon 3 and are used to influence their transmission properties in a manner that establishes a desired operating point. Usually this operating point is the inflection point of the transmission function of the reference Fabry-Perot etalon 3. These electrodes 4 affect the transmission properties through electro-optic or thermo-optic effects. Alternatively, thin film resistors or Peltier elements can be used in place of the tuning electrodes 4. They use thermal effects or thermo-optical effects to thermally change the permeation properties of the reference Fabry-Perot etalons 3.

Another way of changing the transmission characteristics of the reference Fabry-Perot etalon 3 uses the liquid crystal 5. Such a device may be arranged inside or outside the reference Fabry-Perot etalon 3.

The tuning electrodes 4 can also be replaced with a piezoelectric element deformable by current, which will then deform the reference Fabry-Perot etalon 3. Similarly, the liquid crystal 5 may be replaced with an electrically deformable lens if used. However, in some applications (such as optical microphones), it is best to exclude the use of mechanically deformable elements.

In order to prevent the laser light source 1 from being affected by the light feedback, an optical isolator 2 is used between the laser light source 1 and the reference Fabry-Perot etalon 3. This isolator 2 is a combination of a linear polarizing filter and a quarter wave plate.

Light from the reference Fabry-Perot etalon 3 is detected by a light detector 6, for example a PIN diode.

Light from the laser light source 1 is also incident on the measuring Fabry-Perot etalon 7 which is affected by changes in physical parameters (eg air pressure). The exact way in which the measured Fabry-Perot Etalon 7 will interact with the surroundings in order to be affected by the physical parameters will depend on the nature of the physical parameters. In the case of air pressure, the measuring Fabry-Perot etalon 7 will simply have a cavity that is coupled to the air through the hole. This will be clearer below. Light from the measuring Fabry-Perot etalon 7 is also detected by a light detector (not shown), for example a PIN diode. The output signal from this photo detector will depend on the noise from the laser light source 1 and on the variation of the physical parameters to be measured.

A difference signal between the output signal from this photo detector and the output signal from photo detector 6 is generated by difference amplifier 8. By this means, the common mode laser noise from the laser light source 1 is canceled out. From both the reference path (i.e., through the reference Fabry-Perot Etalon 3 and photo detector 6) and the measurement path (i.e., through the measurement Fabri-Perot etalon 7 and its photo detector) To dynamically adjust the DC level, the difference amplifier 8 has a gain stage that dynamically adjusts both output signals with a long time constant.

Light from the laser light source 1 can be affected both etalons 3, 7 via the beam splitter. Alternatively, a double emission laser light source can be used.

In an alternative switching mode, the tuning electrodes 4 and the liquid crystal 5 are omitted. The laser light source 1 is operated with alternating pulses between the measurement path and the reference path, so that successive pulses are one or the other of the reference Fabry-Perot etalons 3 and the measurement Fabry-Perot etalons 7. One (but not both) is incident. In this switching mode, the operating point can be set by adjusting the current of the laser light source 1 power supply. Although the transmission peaks of the measured Fabry-Perot Etalon 7 do not correspond to the transmission peaks of the reference Fabry-Perot Etalon 3, ideal operation for both etalons 3, 7 due to the sequential mode of operation Points can be obtained. During one cycle, the laser light source 1 current is regulated for one etalon 3, 7, and for subsequent cycles, the laser light source 1 current is regulated for the other etalon 3, 7. .

Noise cancellation is still performed by the difference amplifier 8 after detection by the photo detectors. The switching mode is less effective at canceling the laser noise than the continuous mode using the tuning electrodes 4 or the liquid crystal 5. However, computer simulations show that 1 / f noise can be successfully canceled nonetheless. This has the advantage that a more compact device can be manufactured (due to the omission of the tuning electrodes 4 and the liquid crystal 5) and the overall power consumption is further reduced.

As can be seen, this embodiment generates a noise signal (indicative of noise from the laser light source 1) using the reference Fabry-Perot etalon 3 and measures the measured Fabry-Perot etalon 7. It works by subtracting this noise signal from the generated signal to improve the SNR of the measured Fabry-Perot etalon 7. Both etalons 3 and 7 should preferably be operated at the inflection points of their respective transmission functions. At this point, a linear relationship between the measured physical parameters and the light output and a linear relationship between the frequency noise and the light output are achieved. The slope steepness of the etalons 3, 7 does not produce amplitudes in which the measured physical parameters and frequency noise can move too far from the ideal operating point (ie, inflection points of the periodic transmission functions). By selecting the mirror reflectance and the mirror spacing, it can be adjusted. With this embodiment, the frequency and phase variations in the laser light source 1 are prevented from compromising the performance of the device, and even if the laser light source 1 is an unstable laser diode, it is possible to reach quantum or shot noise limits. It is possible.

FIG. 2 shows an optical microphone based on the same principle as the embodiment of FIG. 1. The optical microphone of FIG. 2 has the significant advantage that it can be manufactured without moving parts such as thin films typically required in conventional small microphones. Therefore, it has a compact size and is very robust.

In addition, the influence of the tuning elements 8 is increased due to the small spacing between them. For example, the tuning mechanism may then be proportional to the electric field which is proportional to the spacing between the electrodes 8.

The laser light emitted from the laser light source 10 (e.g., PIN diode) is incident on an optical isolator consisting of a linear polarizing filter 11 and a quarter wave plate 12. Thereafter, light is coupled into the waveguide structure 13, which comprises a measuring path in which the main element is the measuring Fabry-Perot etalon 14, and the main element in the two mirrors 15a, 15b. The light is split so that light is transmitted along the reference path, which is the reference Fabry-Perot etalon formed by.

Tapered waveguide structures 16a, 16b are provided to couple the waveguide structure 13 into the measurement Fabry-Perot etalon 14. These ensure efficient binding into the measured Fabry-Perot etalon 14. Taping reduces the branching of light emerging from the waveguide structure 13. The reduction in branching occurs at dimensions parallel to the plane of the substrate 23, typically lithium niobate, on which the optical microphone is formed.

The reference cavity can be tuned by tuning the tuning electrodes 17 or thin film resistors in the same manner as described in the embodiment of FIG. 1.

Light emitted from the reference Fabry-Perot etalons and measurement Fabry-Perot etalons 14 is incident on the respective photo detectors 18, 19, and a difference signal is obtained by the difference amplifier 20.

Measurement Fabry-Perot Etalon 14 can be realized in two different ways as shown in FIGS. 3A and 3B. In FIG. 3A, a concentric mirror structure 21 (eg, tubular or hollow core fiber) is used. In FIG. 3B, planar parallel etalon is formed by opposing parallel plane mirrors 22.

The laser light source 10 may be operated in a pulsed switching mode as in the embodiment of FIG. 1. Two different switching modes are envisioned based on two duty cycles of different lengths, allowing for high SNR and low SNR operation. In low SNR mode of operation, the duty cycle is lower, resulting in a significant reduction in current consumption.

It is possible to manufacture the optical microphone of FIG. 2 using a silicon on insulator (SOI) technique. The main advantage of the waveguide structure 13 is the possibility of integrating reference paths with small physical dimensions; The reference Fabry-Perot etalon can typically have dimensions of 1 μm to 1 mm (length) and 1 μm (width).

To operate as a microphone, the measurement Fabry-Perot Etalon 14 is acoustically coupled to the surrounding environment (eg air). This is accomplished by forming a hole in the measuring Fabry-Perot Etalon 14, which provides fluid communication between the ambient air and the air-filling cavity in the Measuring Fabry-Perot Etalon 14. Thus, a change in air pressure (eg caused by sound waves) is coupled to the air-filled cavity and affects the index of refraction in the cavity, which is detected as a change in light intensity at the photo detector 19.

One possible method of making an optical microphone similar to that of FIG. 2 (although there is no waveguide structure) is shown in FIG. 4. This is a stacked or layered design in which several thin film optical layers and devices are stacked in contact with one another. As a result, a compact and robust sensor is obtained. In order to omit the waveguide beam splitter used in FIG. 2, a double-sided emission laser diode 30 embedded in the substrate 31 is employed. Such laser diode 30 may be a vertical cavity surface-emitting laser (VCSEL). The VCSEL can be modified by partially removing the substrate to allow light to be emitted from both the front side and the back side, or can specifically produce double sided emission by omitting a portion of the substrate. Another device that can be used for double-sided emission (again, by modification or special manufacturing) is a distributed feedback (DFB) laser diode.

The layers of the laminate may be made of glass, polymer, silicon, or other dielectric layers, depending on the desired mechanical processing properties and the wavelength of light emitted by the laser diode 30. In addition, a combination of materials in the laminate is possible. The layers can be combined using bonding, gluing, or other techniques.

One of the new laser beams is incident on the reference path; Another laser beam emitted from the opposite side is incident on the measurement path. The reference path is (in order of passing) the optical isolation layer 32, the collimating lens 33 with the antireflective coating 53, the first mirror 34 of the reference Fabry-Perot etalon, the spacer element 35 , A second mirror 36 of the reference Fabry-Perot etalon, and a photodetector 37 embedded in the substrate 38. Thus, the reference Fabry-Perot etalons are made of spacer elements 35 that space the first and second mirrors 34, 36, and the two mirrors 34, 36. Each mirror 34, 36 has antireflective coatings 39a, 39b, 40a, 40b.

The measurement path is similar in configuration. This is in accordance with the light isolation layer 41, the collimating lens 42 with the antireflective coating 43, the first mirror 44 of the measuring Fabry-Perot etalon, the spacer element 45, and the measurement. A second mirror 46 of Fabry-Perot etalon, and elements such as a photo detector 47 embedded in the substrate 48. Thus, the measured Fabry-Perot etalon is made of the first and second mirrors 44, 46, and the spacer elements 45 that separate the two mirrors 44, 46. Each mirror 44, 46 has an antireflective coating 49a, 49b, 50a, 50b.

Spacer element 45 has an opening or aperture 51 through which air in cavity 52 is coupled to the surrounding environment. Therefore, the air pressure in the cavity 52 is affected by the pressure change in the surrounding environment. This will then affect the light transmission properties of the measured Fabry-Perot etalons, and the change in air pressure will be detected as a change in the output signal from the light detector 47. Thus, the device responds to sound waves and acts as a microphone.

To influence the transmission characteristics of the reference cavity, tuning electrodes 53a and 53b are used. Their mode of operation is the same as the tuning electrodes 4 described with reference to the embodiment of FIG. 1.

The final layered structure results in a very compact optical microphone, with the two light beams lying on the same spatial axis. Both aspects are desirable for cost-effective, mass-produced compact devices. Therefore, it is very suitable for mobile communication applications.

Those skilled in the art will understand and implement other modifications to the disclosed embodiments when practicing the claimed invention by reviewing the drawings, specification, and appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and “negative article (a or an)” does not exclude a plurality. A single processor or other unit may perform the functions of several items recited in the claims. The simple fact that certain measurements are quoted in different dependent claims does not indicate that a combination of these measurements cannot be used advantageously. The reference sign in the claims should not be construed as limiting the scope.

Claims (14)

In an apparatus each comprising a measurement interferometer 7 and a reference interferometer 3, which are configured to receive light from the same light source 1 and emit light to the respective detectors 6, each having a working point. ,
The measuring interferometer 7 is configured to respond to changes in physical parameters by changing the intensity of light emitted, while the reference interferometer 3 is configured not to respond to changes in physical parameters,
The apparatus further comprises a signal processor for generating a differential output signal in accordance with the respective output signals generated by the detectors (6).
The device according to claim 1, wherein the measurement interferometer (7) and / or the reference interferometer (3) is a Fabry-Perot interferometer.
The device according to claim 2, wherein the measuring interferometer (7) and / or the reference interferometer (3) comprise a pair of spaced mirrors.
Apparatus according to any one of the preceding claims, further comprising a thermal tuning element for tuning the operating point of the measuring interferometer (7) or of the reference interferometer (3).
5. The device according to claim 1, further comprising a tuning electrode (4) for tuning the operating point of the measuring interferometer (7) or the reference interferometer (3). 6.
The device according to claim 1, further comprising a liquid crystal tuning element (5) disposed between said measuring interferometer (7) or said reference interferometer (3) and each detector (6).
7. A light source controller as claimed in any preceding claim, further comprising a light source controller configured to cause the light source 1 to alternately emit light at the first and second wavelengths,
The operating points of the measuring interferometer (7) and the reference interferometer (3) are achieved at the first and second wavelengths respectively.
8. An optical isolator (2) according to any one of the preceding claims, further disposed between the light source (1) and the reference interferometer (3) and having a linear polarizer and a quarter wave plate. Containing device.
The device according to claim 1, wherein the light source is a laser.
10. The device according to claim 9, wherein the laser is a double-emitting laser, and the first and second emission beams are respectively coupled to the measurement interferometer (7) and the reference interferometer (3).
The laser beam of claim 10 wherein the double-emitting laser is a double-sided emission laser diode disposed on a substrate between the first and second layered structures, each of the first and second layered structures being the measurement interferometer 7 and the reference. An apparatus comprising one of the interferometers and two respective mirror layers spaced by respective spacer layers, and detector layers remote from the substrate with respect to the mirror layers.
12. The apparatus according to any one of the preceding claims, wherein the signal processor further comprises an adaptive equalizer for equalizing the average amplitude of each output generated by the detectors (6) over an equalization period.
13. An optical microphone comprising the device according to any one of the preceding claims,
The measuring interferometer (7) is acoustically coupled to the surrounding environment, the reference interferometer (3) is acoustically isolated from the surrounding environment, and the physical parameter is air pressure.
The optical microphone according to claim 10, wherein the measuring interferometer (7) is acoustically coupled with the surrounding environment by a hole in its cavity.

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